Many of Johnson's failed projects have already been mentioned (Garden Bridge, water cannons, etc.).
The Routemaster bus which he also introduced at huge cost is said to have been a failure. They cost twice the amount of the old double deckers, carry fewer passenger (87 instead of 96), were fitted with an ineffcient air cooling system which led to people expiring in the heat and new openable windows had to be fitted, the jump on/jump off feature has been more or less phased out (the prinipal purpose of the Routemaster), hybrid battery system doesn't work properly and most buses are running entirely on diesel.
The Thames Cable Car costing £60 million, £24 million of which funded by taxpayers - it is poorly used since there is little demand for travel from the Greenwich Peninsula to the Royal Docks.
Boris Island Airport Scheme, refused after millions being spent on the project
Stratford Orbital Structure - which many people (me included) think is pointless and ugly - costs £10,000 per week to maintain.
Just a few of Johnson's stupid vanity projects.
I am not a huge fan of Sadiq Khan but I think he is a good deal better than Johnson.